1. Technical Field
The present invention relates to a positive electrode for a nonaqueous electrolyte secondary battery and a production method thereof. Particularly, the present invention relates to a positive electrode for a nonaqueous electrolyte secondary battery using a plurality of positive electrode active materials having different physical properties, which has excellent retention characteristics in a charged state, and is capable of enhancing the capacity and energy density; and a production method thereof.
2. Related Art
With the rapid spread of portable electronic equipment, the specifications required of the batteries used in such equipment have become more stringent every year, and there is particular requirement for batteries that are compact and thin, have high capacity and superior cycling characteristics, and give stable performance. In the field of secondary batteries, attention is focusing on lithium nonaqueous electrolyte secondary batteries, which have high energy density compared with other batteries. These lithium nonaqueous electrolyte secondary batteries are winning an increasingly large share of the secondary battery market.
Here, in an instrument in which such a type of the nonaqueous electrolyte secondary battery is used, since a space in which the battery is held is prismatic (plane box-shaped) in many cases, a prismatic nonaqueous electrolyte secondary battery produced by holding a power element in a prismatic outer can is frequently used. The constitution of such a prismatic nonaqueous electrolyte secondary battery is described with reference to the drawings.
FIG. 1 is a perspective view showing a related-art prismatic nonaqueous electrolyte secondary battery by sectioning the battery perpendicularly. This nonaqueous electrolyte secondary battery 10 is produced by holding a plate wound electrode assembly 14 produced by winding a positive electrode 11, a separator 13 and a negative electrode 12 which are laminated in this order, in the inside of a prismatic battery outer can 15, and by sealing the battery outer can 15 with an opening-sealing plate 16. The wound electrode assembly 14 is wound so that for example, the positive electrode 11 is positioned in the outermost periphery and exposed. The exposed positive electrode 11 in the outermost periphery is directly contacted with the inside of the battery outer can 15 serving also as a positive electrode terminal and is electrically connected. Further, the negative electrode 12 is formed in the center of the opening-sealing plate 16 and is electrically connected to a negative electrode terminal 18 provided through an insulator 17, through a current collector 19.
Further, since the outer can 15 is electrically connected with the positive electrode 11, in order to prevent the short circuit of the negative electrode 12 with the battery outer can 15, an insulating spacer 20 is inserted between the upper terminal of the wound electrode assembly 14 and the opening-sealing plate 16 so that the negative electrode 12 and the battery outer can 15 are in an electrically insulated state to each other. The positions of the positive electrode 11 and the negative electrode 12 are sometimes exchanged with each other. This prismatic nonaqueous electrolyte secondary battery is produced by inserting the wound electrode assembly 14 into the battery outer can 15; by laser-welding the opening-sealing plate 16 to an opening of the battery outer can 15; by pouring a nonaqueous electrolyte through an electrolyte pouring pore 21; and by sealing the electrolyte pouring pore 21. With such a prismatic nonaqueous electrolyte secondary battery, not only is the waste of the space during the use thereof small, but also the excellent advantageous effects of high battery performance and reliability of the battery are exhibited.
As a negative electrode active material used in the nonaqueous electrolyte secondary battery, carbonaceous materials such as graphite and an amorphous carbon are widely used, since carbonaceous materials have such excellent performance such as high safety because lithium dendrites do not grow therein while they have a discharge potential comparable to that of lithium metal or lithium alloy; excellent initial efficiency; advantageous potential flatness; and high density.
Further, as a nonaqueous solvent of a nonaqueous electrolyte, carbonates, lactones, ethers and esters are used individually or in combination of two or more thereof. Among them, particularly carbonates having a large dielectric constant and having high ion conductivity thus the nonaqueous electrolyte thereof are frequently used.
On the other hand, as a positive electrode active material, lithium transition-metal compound oxide such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), spinel-type lithium manganese oxide (LiMn2O4) and lithium iron oxide (LiFeO2) is used, because it is known that by using such a positive electrode in combination with a negative electrode composed of a carbon material, a 4-V-class nonaqueous secondary battery having a high energy density can be obtained. Among them, particularly because of more excellent battery performance than those of other materials, lithium cobalt oxide and different metal elements-added lithium cobalt oxide are frequently used, however, since not only is cobalt expensive, but also the existing amount of cobalt as a resource is small, for continuous use of lithium cobalt oxide as a positive electrode active material of the nonaqueous electrolyte secondary battery, it is desired to make the nonaqueous electrolyte secondary battery have even higher performance and longer life.
For making the nonaqueous electrolyte secondary battery in which lithium cobalt oxide is used as a positive electrode active material, having even higher performance and longer life, it is an essential task to enhance the capacity and energy density of the battery and improve the safety of the battery. Among them, as a method for enhancing the capacity of the battery, enhancing the density of an electrode material, making a current collector and a separator thinner, and enhance the charging voltage of the battery, are generally known. Among them, enhancing the charging voltage of the battery is a useful technology as a method capable of realizing the enhancing of the capacity without changing the constitution of the battery and is an essential technology for enhancing the capacity and the energy density of the battery.
For example, in a nonaqueous electrolyte secondary battery using the lithium-containing transition metal oxide such as lithium cobalt oxide as a positive electrode active material and using a carbon material as a negative electrode active material, when the positive electrode is used in combination with a negative electrode active material of a carbon material such as graphite, the charging voltage is generally 4.1 to 4.2 V, while the potential of the positive electrode active material is 4.2 to 4.3 V based on lithium. Under such a charging condition, the capacity of the positive electrode is utilized in only 50 to 60% relative to a theoretical capacity. Therefore, when the charging voltage can be enhanced more, the capacity of the positive electrode can be utilized in 70% or more relative to the theoretical capacity and enhancing the capacity and energy density of the battery becomes capable.
For example, JP-A-9-306546 discloses an invention of a positive electrode for a nonaqueous electrolyte secondary battery by which enhancing the capacity and energy density of the nonaqueous electrolyte secondary battery was contemplated by enabling high density charging through the use of two types of positive electrode active material having differing average particle diameters and through the use of composite particles produced by coating the surface of a positive electrode active material having the larger average particle diameter among the above different average particle diameters with a positive electrode active material having the smaller average particle diameter. Further, JP-A-2004-127694 discloses an invention of a positive electrode for a nonaqueous electrolyte secondary battery by which enhancing the capacity and energy density of the nonaqueous electrolyte secondary battery was contemplated by causing a high density charging to be able through using composite particles produced by coating the surface of a LiNiAlO2-based positive electrode active material having a larger average particle diameter with a LiNiCoMnO2-based positive electrode active material having a smaller average particle diameter.
On the other hand, JP-A-2005-317499 discloses an invention of a nonaqueous electrolyte secondary battery using a mixture of lithium cobalt oxide and layer-shaped lithium nickel cobalt manganese oxide to which a different metal element is added as a positive electrode active material, and capable of being stably charged at a high charging voltage. This positive electrode active material is produced so that by adding different metal elements of at least Zr, Mg to lithium cobalt oxide, the structural stability thereof at a high voltage (to 4.5 V) is improved and further, by incorporating layer-shaped lithium nickel cobalt manganese oxide having high thermal stability at a high voltage, the safety is secured. By using a combination of a positive electrode using the above positive electrode active material and a negative electrode having a negative electrode active material composed of a carbon material, a nonaqueous electrolyte secondary battery capable of being stably charged at a high charging voltage of 4.3 V or more and 4.5 V or less (the final positive electrode charging voltage is 4.4 V or more and 4.6 V or less based on lithium), has been obtained.
As described above, various improvements for enhancing the capacity and the energy density of the nonaqueous electrolyte secondary battery containing lithium cobalt oxide as a positive electrode active material have been performed. However, in the inventions of the positive electrode for the nonaqueous electrolyte secondary battery disclosed in JP-A-9-306546 and JP-A-2004-127694, there is such a problem that since a positive electrode active material having a smaller average particle diameter has a large reactivity, that the positive electrode active material having a smaller average particle diameter is selectively deteriorated earlier during charging/discharging, so that not only does the battery blister occur due to the generation of a gas, but also the deterioration of the cycle performance is large.
Further, according to the invention disclosed in JP-A-2005-317499, there is such a problem that particularly in a high charging voltage region, though the deterioration of lithium nickel cobalt manganese oxide is small, the deterioration of lithium cobalt oxide to which a different metal element is added is rapidly deteriorated, so that like the above description, not only does the battery blister occur due to the generation of a gas, but also the deterioration of the cycle performance is large.